The present invention relates to sensor switches and, more particularly, to misgrip sensor switches used in combination with arms supporting a workpiece during successive forming operations.
In many manufacturing operations, of which a stamping operation is typical, support arms are used to engage and hold a workpiece, which may begin the operation as a flat sheet of formable material such as steel. The support arms hold the workpiece loosely between upper and lower tools, such as a forming die set. Since a single workpiece of sheet metal may be conveyed to successive die sets for a complete forming operation, it is necessary that the support arms hold the workpiece continuously, conveying it between the successive die sets on a line.
Since such forming operations are highly automated, it is necessary for the controller which operates the support arms to detect whether a sheet workpiece is not picked up by the support arms, is misregistered on the support arms, or falls from the support arms between successive forming operations. Consequently, such support arm systems typically include misgrip sensors. The typical misgrip sensor includes a contact arm having an outer head positioned to be engaged by a workpiece when properly engaged by the support arms and a housing slidably receiving the contact arm. The contact arm includes a body having a recess positioned to receive the end of a microswitch mounted within the housing.
Displacement of the contact rod a predetermined amount enables the microswitch end to engage the recess in the contact rod body, thereby closing the switch which generates a signal indicating proper engagement of the workpiece by the arms. Improper engagement of a workpiece by the support arms does not displace the rod sufficiently, if at all, so that the switch remains in an open position indicating an error condition.
A disadvantage with such types of switches is that the microswitch can stick, thereby giving a false reading of misalignment, or a threshold condition may occur where the workpiece is properly aligned but the contact rod is not displaced sufficiently to enable the microswitch to fully engage the recess, also creating a false error condition. Accordingly, there is a need for a misgrip sensor which detects proper engagement of a workpiece by support arms with a minimum of false readings.